1. Field of the Invention
This invention relates to telecommunications networks. More particularly, the invention relates to a method of transferring data in an ATM (Asynchronous Transfer Mode) network and of dynamically changing the length of a structured data transfer (SDT) in such ATM network.
2. Description of the Prior Art
In a digital transmission network, data from a large number of users are serially transmitted from one network node to another network node, up to their respective final destination.
Due to the evolution of networks towards more and more complex mixing of sub-networks with heterogeneous architectures, it is clear that there is a requirement to support distributed computing applications across high speed backbones that may be carrying LAN traffic, voice, video and traffic among channel-attached hosts and workstations.
Fast packet switching is now commonly used to accommodate the bursty, multiprocess communications found in distributed computing environments.
Recently the concept of cell switching has been introduced. Cell switching can be thought of as a high performance form of packet switching. In packet switching networks, the packet size is a fixed maximum, but individual packets may always be shorter than the maximum. In a cell based network, cells have a fixed length. Cells are usually a lot shorter than packets, because the use of short fixed length cells simplifies the hardware needed in each node of the network.
Asynchronous Transfer Mode (ATM) is a protocol for user access to the internal operation of a public high speed cell switching system. This protocol is suitable for all kinds of traffic: data, voice, image, video.
In order to make an ATM network practical, it is necessary to adapt the internal network characteristics to those of the various traffic types that will use the network. This is the purpose of the ATM Adaptation Layer (AAL). The function of the AAL is thus to provide generalized interworking across the ATM network. The AAL function operates an end-to-end protocol across the ATM network to provide support for end users of different classes of service corresponding to generic classes of network traffic.
One of these classes (Class One) is intended for constant rate voice and video applications. Class one requires the following environment, in which the present invention finds applications: a constant bit rate at source and destination; a timing relationship between source and destination, and the transfer of structured information between source and destination.
Communication methods that satisfy these requirements are disclosed in Revised Recommendation 1.363 from CCITT, which is included herein by reference.
This Recommendation describes the interactions between the AAL and the next higher (OSI) layer, and between the AAL and the ATM layer (sub-layer of layer 1). The AAL isolates the higher layers from the specific characteristics of the ATM layer by mapping the Protocol Data Units (PDUs) of the higher layers into the information field of the ATM cell and vice versa. The AAL entities exchange information with the peer AAL entities to support the AAL functions.
To support services above the AAL, some independent functions must be performed in the AAL. These functions are organized in two logical sublayers, the Convergence Sublayer (CS) and the Segmentation and Reassembly sublayer (SAR).
The SAR primary functions are segmentation of higher layer information into a size suitable for the information field of an ATM cell and reassembly of the contents of ATM cell information fields into higher layer information.
The CS primary function is to provide the AAL service at the AAL Service Access Points (SAP).
The SAR sublayer at the transmitting end accepts a 47- byte block of data (SAR.sub.13 PDU payload) from the CS sublayer and then adds a one byte SAR.sub.-- PDU header to each block to form the SAR.sub.-- PDU.
The SAR sublayer at the receiving end receives the 48- byte block of data from the ATM layer and separates the SAR.sub.-- PDU header from the data payload. The 47-byte block of data of the SAR.sub.-- PDU payload is then passed to the CS sublayer. The basic AAL1 header is 1 byte long and the payload is 47 bytes long.
One problem arising in a data transfer environment, as described above, is the need to change the constant bit rate of the data that are being exchanged between a transmitting end and a receiving end. Such a need can be inferred from the real time requirements of some multimedia services, where the bit rate should not be constant anymore, but should vary over time according to user demand. An example of such a need is expressed in ITU-T contribution Com 13 D-81 of July 93, by J. Y. Cochennec: . . . "after the connection with the server has been established, the user may typically ask for a video sequence, then for commentary, then for audio only, etc. Each time the user formulates a request to the server, the received bit rate may vary, but within a sequence the bit rate will be constant." This means that the bit rate should be dynamically modifiable, while the connection is active.
A similar issue has already been addressed in European Patent Application 0 214 352 A1, published Mar. 18, 1987 relative to a dynamic bandwidth allocation mechanism between circuit slots and packet bit stream in a communication network. This patent application describes a method using a signalling channel and a data channel with associated in-band control. According to the published method, the control is specified in-band (i.e. in the data channel, with only a minimum of associated control information) whether a slot should be added or suppressed, and which slot is impacted. However, this method is not compatible with the AAL1 format, since in AAL1 there is no field containing both the slot add/suppress information and the slot number information. Further, this method is limited to adding or suppressing one slot per call operation. In addition, the source which initiates the change of data structure does not know whether the change using the in-band signalling protocol will be accepted by the destination. Therefore, call contentions are detected after the inband signalling protocol is started. This makes difficult the increase/decrease in the bandwidth of an existing online connection, and guaranteeing the data integrity under the bit rate after a change.